A wide variety of implantable heart monitors and therapy delivery devices have been developed including pacemakers, cardioverter/defibrillators, heart pumps, cardiomyostimulators, ischemia treatment devices, and drug delivery devices. Most of these cardiac systems include electrodes for sensing and sense amplifiers for recording and/or deriving sense event signals. Often the sense event signals are utilized to control the delivery of the therapy in accordance with a predefined algorithm.
Implantable pulse generators are well known in the prior art. Most demand pacemakers include sense amplifier circuitry for detecting intrinsic cardiac electrical activity so that the devices may be inhibited from generating unnecessary output electrical stimulating pulses when a heart is functioning properly. Dual-chamber cardiac pacemakers typically have separate sense amplifiers for atrial and ventricular sensing. The sense amplifiers detect the presence of intrinsic signals, such as P-waves occurring naturally in the atrium and R-waves occurring naturally in the ventricle. Upon detecting an intrinsic signal, sense amplifier circuitry generates a digital signal for output to other components which inhibit the delivery of a pacing pulse to the corresponding chamber.
It is desirable to accurately and reliably measure the response of the heart to an electrical stimulating pulse. Measuring such a response permits the determination of a patient's stimulation threshold, or the minimum energy a stimulating pulse must contain for a cardiac response to be evoked. Once a patient's stimulation threshold is determined, the energy content of stimulating pulses may be adjusted to avoid delivering pulses having unnecessarily high energy content. Minimizing the energy content of stimulating pulses is believed to have physiological benefits, and additionally reduces power consumption, a key concern in the context of battery-powered implantable devices.
Detection and measurement of the response of the heart to an electrical stimulating pulse may also be useful in controlling a pacemaker's pacing rate, for ascertaining the physiological effect of drugs or for diagnosing abnormal cardiac conditions.
Immediately following delivery of a pacing pulse to cardiac tissue, a residual pace polarization artifact (also called a post-pace polarization artifact or a pace polarization signal) is generated by the charge induced in the electrode tissue interface by delivery of a pacing pulse. If the pacing pulse captures the heart and causes an evoked response in the cardiac tissue, then an evoked response signal is superimposed atop the typically much larger amplitude pace polarization artifact. As a result, conventional pacemakers or pacemaker-cardioverter-defibrillators (“PCD's”) either cannot differentiate, or have difficulty differentiating, between pace polarization artifacts and evoked response signals. This problem is further complicated and exacerbated by the fact that residual pace polarization artifacts typically have high amplitudes even when evoked response signals do occur. Consequently, it becomes difficult, if not impossible, to detect an evoked response signal using a conventional pacemaker or PCD sense amplifier employing linear frequency filtering techniques. As a result, most pacemakers cannot discern between pace polarization artifacts and evoked response signals.
Most pacemakers employ sensing and timing circuits that do not attempt to detect evoked response signals until the pace polarization artifact is no longer present or has subsided to some minimal amplitude level; only then is sensing considered reliable. In respect of capture detection, where the pacemaker detects whether the pacing pulse to the cardiac tissue evoked an effective stimulated response, such sensing typically occurs a significant period of time after the evoked response signal has already occurred. As a result, most pacemakers cannot detect evoked response signals with any degree of confidence.
The generation and delivery of an electrical pulse gives rise to the storage of charge in the electrode tissue interface. Such charge storage produces pace polarization artifacts, which typically have much larger amplitudes than those corresponding to electrical signals arising from an intrinsic heartbeat or a stimulated response. Pace polarization artifacts may also interfere with the detection and analysis of a stimulated or evoked response to a pacing pulse. Thus, a need exists in the medical arts for determining reliably whether or not an evoked response signal has occurred in a pacing environment.
Pace polarization artifacts typically arise due to the electrode-tissue interface storing energy after a pacing stimulus has been delivered. There are typically two electrode-tissue interfaces in a pacing circuit: one for the tip electrode, and one for the ring (or canister) electrode. The stored energy dissipates after the pace event, creating the after-potential.
In respect of the impedance sensed by a pacemaker's internal circuitry, the total load of the pacing circuit comprises the impedance of the lead itself, the electrode-tissue interface impedances, and the impedance of the body tissue bulk. The impedances of the body tissue and the lead may be modeled as a simple series bulk resistance, leaving the electrode-tissue interfaces as the reactive energy absorbing/discharging elements of the total load. The tip electrode is the primary after-potential storage element in comparison to the case and ring electrodes. In a pacing circuit, a ring electrode typically stores more energy than does a case electrode due to differences in electrode areas.
Several methods have been proposed in the prior art for improving an implantable device's ability to detect and measure evoked responses. For example, U.S. Pat. No. 5,172,690 to Nappholz et al., entitled “Automatic Stimulus Artifact Reduction for Accurate Analysis of the Heart's Stimulated Response,” hereby incorporated by reference herein its entirety, proposes a tri-phasic stimulation waveform consisting of precharge, stimulus, and postcharge segments. The duration of the precharge segment is varied until the amplitude of the pace polarization artifact is small compared to the evoked response.
Other disclosures related to the same general subject matter include the U.S. Patents listed below in Table 1.
TABLE 1CountryU.S. Pat. No.Inventor/ApplicantIssue DateU.S.A.4,644,954Wittkampf et al.Feb. 24, 1987U.S.A.4,972,834Begemann et al.Nov. 27, 1990U.S.A.5,954,756Hemming et al.Sep. 21, 1999
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description and claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.